At present, the heating chamber of every commercial microwave oven is composed of a top, a bottom, a back wall, two side walls and an oven door, all of which being of flat plate type. Because the launching position of the magnetron in the microwave oven is fixed, the reflection angle of microwave emitted by the magnetron formed by a reflection of the flat panel wall of the heating chamber is substantially fixed, therefore, the microwave distributes in every space point in a certain and regular way, i.e. the microwave at one space point is different from that of another, and the heating effect at a space point where more microwave is obtained is better than that of a space point where less microwave is obtained. Although the rotating tray has the effect of making the food thereon heated evenly, some parts of the food still cannot be heated sufficiently by getting more microwave because the distribution of the microwave in the heating chamber is uneven.